One of the simplest and yet very widely used flowmeters is the rotameter, which comprises a vertically disposed tapered flow passage with cross section progressively increasing from the bottom inlet to the top outlet and a flow obstructing member suspended in the fluid stream by the drag force exerted by the upwardly moving fluid, wherein the flow rate of the fluid is determined from the vertical position of the flow obstructing member that is lifted by the drag force counter-acting the weight thereof. The present-day rotameter has a serious deficiency: The lack of a read-out device that provides flow rate in the form of electrical signals that can be fed into a flow computer or other flow data processing device. There are a few rotameters available today which employ one or another form of transducer converting the position of the flow obstructing member to an electrical signal, but the accuracy and reliability of these transducers is quite marginal at best and most of the rotameters in use today have only visual read-out means such as a scale or dial. Since an overwhelming majority of flowmeters employed in industrial, commercial and medical applications constitute elements in automatic process control, it is imperative that the flowmeters must have an electrical form of read-out that can be fed into flow data processors and/or flow control devices. Another deficiency of the present day rotameters is that every individual rotameter has to be calibrated for the specific fluid under measurement, as the force counter-balanced by the drag force of the fluid flow does not remain constant due to the buoyancy force experienced by the flow obstructing member in the rotameter, which buoyancy force changes as a function of the fluid density. A further deficiency of the existing rotameters is their inability to measure the volume flow rate of fluid directly, as they measure the drag force as a measure of the fluid flow instead of the volume or mass flow rate of fluid.